1. Field of Invention
The present invention relates to a method of making a high quality optical waveguide substrate of which the surface is oxidized from a silicon substrate through relatively large thickness and has no foreign matter particles adhered to the surface.
2. Description of Related Art
A waveguide device for optical communication is constituted of an optical waveguide and a semiconductor integrated circuit. The optical waveguide is formed on a quartz substrate. The quartz substrate is such that a quartz film, i.e. silicon dioxide, is formed on the surface of a silicon substrate. Since this film functions optically as an under-clad of the optical waveguide, it is required to be as thick as 5-30 xcexcm.
On the other hand, the semiconductor integrated circuit is formed on a silicon substrate. A thickness of an oxidation film required for the semiconductor integrated circuit is 0.2-3 xcexcm, which is much thinner than the quartz film for the optical waveguide.
Since silicon constituting a substrate has strong affinity with oxygen and is easily oxidized. a technique to form a quartz film by oxidation on the silicon surface of the substrate is adopted as a method for forming such oxidation film of the semiconductor integrated circuit and quartz film of the optical waveguide.
Such techniques are, for example, dry oxygen oxidation method, wet oxidation method, steam oxidation method, hydrogen burning oxidation method and hydrochloride oxidation method. The wet oxidation method, the steam oxidation method and the hydrogen burning oxidation method are able to form a quartz film through oxidation of the silicon surface by applying high reaction property oxygen generated by thermal decomposition of steam, in which the silicon substrate placed on a carbon contained ceramics sample base inside a high thermal resistance carbon contained ceramics furnace core tube is heated to a high temperature while in contact with steam. Oxidation rates of those methods are relatively high.
FIG. 2 shows the relationship between the thickness of the oxidation film and the oxidation time when oxidizing the silicon surface in the steam oxidation method, relative to various oxidation temperatures, i.e. heating temperatures of the silicon surface. As seen in FIG. 2, to form an quartz film of 0.2 to 3 xcexcm using for a semiconductor integrated circuit, it takes 10 to 1000 minutes at 1200xc2x0 C. Usually, to obtain such homogeneous thin quartz film formed in a short time, the silicon substrate is exposed to inert gas atmosphere while heating temperature is raised to the oxidation temperature. Once it reaches the oxidation temperature the silicon substrate is in contact with steam.
If the quartz film of 10 to 25 xcexcm generally used for optical waveguide is to be formed in the same manner, it takes 20000 to 125000 minutes at 1200xc2x0 C., which is a considerable amount of time. It is at this point that foreign matter particles adhere to the surface of the quartz substrate. Such foreign matter particles amplified through an electron microscope can be observed as a brown needle like foreign matter.
The development mechanism of the particle is assumed as follows. For instance, if a sample base or a furnace core tube made of silicon carbide is used, the silicon carbide on the surface of the sample base exposed to high temperature for a long time may be partially oxidized and form a quartz film having a thickness of several microns. When the sample base, for instance, is repeatedly used and is exposed to inert gas atmosphere at a high temperature, the silicon carbide inside operates as a reducer to the quartz film and SiO having a subliming property will generate as shown in the following chemical formula:
3SiO2+SiCxe2x86x924SiO+CO2
It is assumed that the SiO, after being sublimated and scattered, adhered to the surface of the silicon substrate and was gradually grown to become foreign matters. Since such consequence attributes to the reductive action by carbon, same results may be obtained in general when using ceramics containing carbon as a sample base or a furnace core tube.
The adhered foreign matter particles may become optical diffusion spots, or cause optical loss, and consequently, may not only degrade the performance of the optical waveguide substrate but also make poor yield.
The object of the present invention developed to solve the foregoing problem, is to provide a method of making a high quality optical waveguide substrate, in which the surface of the silicon substrate is oxidized through relatively large thickness and is free from any foreign matter particles adhering on its surface.
The method of making an optical waveguide substrate according to the present invention, developed to achieve the foregoing object, is such that a silicon substrate to form a quartz film for optical waveguide is placed on a carbon contained ceramics sample base and is inserted into a furnace core tube made of carbon contained ceramics, of which a heating furnace is arranged to its exterior, and an oxidant gas for the silicon surface of the silicon substrate is introduced into the furnace core tube which is heated up to 1200 to 1350xc2x0 C. by the heating furnace to thus oxidize the silicon surface. It begins to introduce the oxidant gas into the furnace core tube while increasing the temperature to from 1200 to 1350xc2x0 C.
It is preferable to begin introducing the oxidant gas into the furnace core tube when the inside of the furnace core tube is heated to 200 to 600xc2x0 C. as the temperature is increased.
During the introduction of the oxidant gas, if the temperature of the furnace core tube is less than 200xc2x0 C., foreign matter particles may easily adhere to the silicon substrate by, for instance, static electricity, while if the temperature is higher than 600xc2x0 C., the reaction as in the afore mentioned chemical formula may occur.
In order to oxidize the silicon surface, if the temperature of the furnace core tube is less than 1200xc2x0 C., it takes long to form a thick quartz film, thus is inefficient. If the temperature is higher than 1350xc2x0 C., defects such as slips tend to occur in the silicon substrate.
The termination of the oxidation of the silicon surface is preferably implemented by continuously introducing the oxidant gas until the temperature inside the furnace core tube is dropped to 200 to 600xc2x0 C. The introduction of the gas is terminated after the temperature inside the furnace core tube is cooled to the above temperature.
It is preferred that the oxidant gas comprises steam or oxygen. It is possible to introduce a simple substance of steam into the furnace core tube, or generate steam by introducing oxygen gas and hydrogen gas into the furnace core tube. Moist oxygen gas or high-pressure oxygen that has gone under high partial pressure may be introduced as well.
It is preferably implemented that the ceramics containing carbon is a silicon carbide.
According to this method, when the furnace core tube is at high temperature, the inside thereof is filled with oxidant gas. Under this oxidant gas atmosphere, carbon contained in the ceramics constituting the furnace core tube and the sample base is restrained from acting as a reducer. As a result, the reduction reaction shown in the foregoing chemical formula hardly happens, therefore, no SiO is produced, and consequently, no foreign matter particles adhere to the surface of the silicon substrate.